Bob Tellefsen wrote:
Hi Fred
Any antenna heavily loaded with coils will show a narrow bandwidth.
In effect, the coils and the capacitance of the antenna elements
across them form a tuned circuit. The higher the Q of the tuned
circuit (meaning the lower the coil losses) the narrower the
bandwidth, but the higher the efficiency. If you find a loaded
antenna with a large bandwidth, it is by definition much less
efficient due to the inherent losses.
My BP is quite narrow-band, and the BW decreases as one goes to lower
bands [more of the equivalent circuit of the overall antenna resides in
the lumped, hi-Q inductance I guess]. It'll cover the nominal CW
portion of the higher bands, 30m --> 10m if I adjust it for the middle
of the sub-band. On 40, if I adjust it with the MFJ 259 at 7025, it'll
be under 1.5:1 from 7000 to maybe 7040 or so, but the VSWR rises rapidly
above that. I don't have the 80m coils, but I would suspect it would be
VERY narrow that low in freq. I'm not sure just how useful an antenna
as small as a BP would be on 80m at QRP levels.
There really is no free lunch.
The B&W "all band folded dipole" exhibits acceptable VSWR 3-30 MHz,
which sounds like free food. It also includes a resistor which forms
the major source of the feed impedance [and that eats a chunk of your
power] so the "lunch" turns out not to be quite free after all. In
fact, it's common for dummy loads to exhibit a phenomenal VSWR bandwidth.
Sometimes you can fudge a bit by tuning such an antenna to
the center of a band, and using an autotuner like the Elecraft
T1 at the feedpoint.
Note, Bob says "at the feedpoint," which would be up at the VersaTee.
Using an autotuner in the radio [e.g. KAT2 or KAT100] gives 1.0:1
matches but the losses go up in the coax.
I do make a lot of Q's on my BP with the K2 and KX1. Biggest thing I've
found is to get the antenna up in the air and away from other objects.
The threads on the BP mast fit painter's poles which is what I use.
This July will be my first FOTBB with the BP. I'm anxious to see if it
beats the 27' wire in the tree.
73,
Fred K6DGW
- Northern California Contest Club
- CU in the 2007 CQP Oct 6-7
- www.cqp.org
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